Blades of modern wind turbines show a length of 45 meters or more. The blades are optimized in view to their weight, in view to their aerodynamic characteristics and in view to the most possible generation of electrical power, which is based on the blade-properties.
Thus the optimizations sooner or later will result in quite flexible and long blades. Due to the flexibility of the blades the problem arises that the tip ends of the turning blades of a wind turbine in operation might hit the tower.
The distance between the tower and the tip end of the blade is also called “blade tip/tower clearance”.
It is known to measure the deflection of a turning blade by monitoring the tip-end of the blade.
One possibility is to attach strain gauges on the blade-surface. They are mainly used for test purposes to gather a certain knowledge about the blade when it is stressed by the rotational movement. The installation of gauges and the installation of a respective bus-system for the transmission of measured data are quite expensive and time extensive and elaborated. As the whole equipment of the system might be exposed to lightning strikes the whole system is inappropriate for a longer period of time.
Document DE 10 2006 054 667 A1 describes a warning system, which is arranged to prevent that turning rotor-blades of a wind turbine hit the wind turbine tower. The bending of a turning blade is determined by sensors. The sensors perform optical measurements, ultra-sonic measurements or RADAR-based measurements, which are used to determine the bending of the blade.
Document EP 2 485 011 A1 describes an arrangement to measure the deflection of a wind-turbine blade. A passive reflector is arranged at the tip end of the blade. An antenna-system is arranged at the top of the nacelle and close to the root end of the blade. The antenna system is built as a linear antenna system and contains a transmit antenna and a receive antenna. The passive reflector and the antenna-system are coupled by a radio signal, which is sent from the transmit antenna via the reflector towards the receive antenna. The receive antenna is connected with an evaluation unit, which is prepared to measure the deflection of the blade based on determined phase-angles of the received radio signal.
The solutions as described in DE 10 2006 054 667 A1 and in EP 2 485 011 A1 result in a “blade tip/tower clearance”, which is not accurate enough as these solutions trust in passive elements like reflectors and sensors.